The present invention relates to methods and apparatuses for receiving changed service-related information conveyed by a signal that is transmitted by a network component of a mobile communication system.
The technology discussed herein relates generally to mobile communication systems. In a mobile communication system, User Equipment (UE) is capable of maintaining its communication service while moving throughout a geographical coverage area of the system. To enable this capability, the system's coverage area is provided with a number of geographically separated base stations that serve as the UE's portal to the mobile communication system. The UE always sends data to, and receives data from, the mobile communication system via a base station. In a typical system, the UE is connected in both the uplink and downlink directions to the base station having the most favorable radio conditions. The area covered by the base station is usually called a cell, and the cell to which the UE is connected is usually referred to as a serving cell.
To achieve compatibility and interoperability between UE's made by various manufacturers, as well as to avoid causing disturbing interference to unrelated devices, mobile communication systems typically need to comply with various standards and government regulations. A number of these are used and well-known in the art. To facilitate this discussion, terminology and network configurations that comply with the Universal Mobile Telecommunication System (UMTS) standard are used herein because these are known and will be readily understandable to the person of ordinary skill in the art. However, the use of this terminology and these configurations is done solely for the purpose of example rather than limitation. The various inventive aspects to be described in this document are equally applicable in many different mobile communications systems complying with different standards.
In the past, cellular systems have focused mainly on transmission of data intended for a single user. However, cellular networks have recently started to introduce additional services such as Multimedia Broadcast and Multicast Services (MBMS), which was introduced for Wideband Code Division Multiple Access (WCDMA) in Release 6. MBMS provides both point-to-point and point-to-multipoint multimedia services in which the same data (e.g., text, audio, picture, video) is transmitted from a single source to multiple users. An exemplary topology of such network is depicted in FIG. 1, in which the Broadcast/Multicast Service Center (BM-SC) serves as entry point for the services. A stream of data for the various services is provided by the BM-SC, and flows through a Gateway GPRS Support Node (GGSN) and Serving GPRS Support Node (SGSN) to a Radio Network Controller (RNC). Based either on a static configuration or on the number of UEs that are interested in receiving the service (derived by a procedure called counting), the RNC decides whether the service will be broadcast from a NodeB to multiple UEs as in Cell A of FIG. 1 (point-to-multipoint—“PTM”) or whether the service will be sent to only one or a limited set of UEs by means of dedicated point-to-point (PTP) transmissions, as illustrated for Service X in Cell B, and also for Service Y in each of Cells B and C.
In MBMS-enabled UMTS systems, application data to be conveyed by means of a PTM transmission is carried on a logical channel called the MBMS Traffic Channel (MTCH). Control information (e.g., what services are currently available, in which mode (PTM or PTP) they are available and other configuration information) is broadcast on a logical channel called the MBMS Control Channel (MCCH). The MTCH and MCCH are both conveyed by means of a Secondary Common Control Physical Channel (S-CCPCH). Another physical channel, called the MBMS Indicator Channel (MICH), is used to convey information from the network to inform the UEs about the information changes on the MCCH. The MCCH and MICH are both conveyed as part of a radio frame, as illustrated in FIG. 2.
Looking at this mechanism in more detail, transmission on the MCCH follows a fixed schedule, as illustrated in FIG. 3. The MCCH information is transmitted using a variable number of consecutive Transmission Time Intervals (TTIs). In each of a number of modification periods, critical information remains unchanged and is periodically transmitted based on a repetition period. This means that a UE needs to receive only one of the multiple transmissions in each modification period to obtain the critical information conveyed during that modification period. To reduce UE power consumption and avoid having the UE constantly receive the MCCH, the MICH conveys information that informs UEs about upcoming changes in the critical MCCH information. In each 10 ms radio frame, 18, 36, 72, or 144 MBMS indicators (also called alarm indicators) can be transmitted, where a notification indicator is a single bit, transmitted using on-off keying and related to a specific group of services.
By relying on the MICH, UEs can sleep and briefly wake up at predefined time intervals to check whether a notification indicator has been transmitted. If the UE detects a notification indicator for a service of interest, it reads the MCCH during the next modification period to find the relevant control information. If no relevant notification indicator is detected, the UE may sleep until the next MICH occasion.
To illustrate the point, FIG. 3 illustrates three modification periods, denoted 1, 2, and 3. During Modification period 1, the MCCH is repeatedly transmitting control information (indicated by the single cross hatching). During that same modification period, the MICH is, in this example, transmitting a notification indicator informing that critical information on the MCCH will change during the next modification period. Accordingly, during modification period 2, the control information on MCCH is different from that which had previously been transmitted (as indicated by the crisscross hatching). If this pertains to a service of interest, a UE should read this information during one of the repeated transmissions.
Also during modification period 2, the MICH indicates that no change to critical information on the MCCH will be made for a service of interest during the next modification period. Accordingly, during modification period 3 the MCCH continues to repeatedly transmit the same information as had been transmitted during modification period 2. Since the UE knows that there is no new information to be obtained during modification period 3, it can refrain from reading the MCCH during modification period 3.
According to recommendations in the Third Generation Partnership Project (3GPP) specifications (TS 25.346 and 25.304) it is assumed that the modification period during which the network continuously transmits MICH frames with the same content will be long enough for the UE to be able to read notification indicators reliably during its regular discontinuous paging occasions (hereafter called “DRX cycle”).
Further according to the 3GPP specifications (TS 25.304 and TS 25.211), when the UE wakes up during its DRX cycle and monitors the MICH, it has to monitor one 16-bit notification indicator (NI) for each MBMS service that it has subscribed to. This is a service that the UE is “interested in.” In other words, for each service, a corresponding notification indicator will be set continuously through the entire length of the modification period preceding a change in the associated MCCH information. The specific value assigned to the 16 bits of the notification indicator is calculated as a function of the identity of the service (e.g., the Temporary Mobile Group Identity—“TMGI”—of the service). The specific mapping is given by the following:NI=(TMGI+└TMGI/G┘)mod G, where G=216.
The number of possible services outnumbers the number of notification indicators that can be transmitted in one MICH frame. Specifically, in 3GPP-compliant systems, each MICH frame contains 288 bits {b0, . . . , b287} which are logically partitioned into groups of 16, 8, 4, or 2 bits. This enables n 1-bit alarm indicators {A0, . . . , An-1} to be mapped onto the MICH bits, where n=18, 36, 72, or 144.
The notification indicator is associated with an index q of the transmitted alarm indicator Aq, where q is computed as a function of the 16-bit notification indicator, the System Frame Number (SFN) of the Primary Common Control Physical Channel (P-CCPCH) radio frame during which the start of the MICH radio frame occurs, and the number of notification indicators per frame (n):
  q  =      ⌊                  (                              (                          C              ×                              (                                  NI                  ⊕                                      (                                                                  (                                                  C                          ×                          S                          ⁢                                                                                                          ⁢                          F                          ⁢                                                                                                          ⁢                          N                                                )                                            ⁢                      mod                      ⁢                                                                                          ⁢                      G                                        )                                                  )                                      )                    ⁢          mod          ⁢                                          ⁢          G                )            ×              n        G              ⌋  where G=216, C=25033.
The mapping from {A0, . . . , An-1} to the MICH bits {b0, . . . , b287} in accordance with TS 25.211 is shown in the following table:
Number of Alarmsper frame (n)Aq = 1Aq = 0n = 18{b16q, . . . , b16q+15} ={b16q, . . . , b16q+15} ={1, 1, . . . , 1}{0, 0, . . . , 0}n = 36{b8q, . . . , b8q+7} ={b8q, . . . , b8q+7} ={1, 1, . . . , 1}{0, 0, . . . , 0}n = 72{b4q, . . . , b4q+3} ={b4q, . . . , b4q+3} ={1, 1, 1, 1}{0, 0, 0, 0}n = 144{b2q, . . . , b2q+1} ={b2q, . . . , b2q+1} ={1, 1}{0, 0}In this mapping, a value of “Aq=1” indicates an upcoming change in the next modification period, whereas a value of “Aq=0” indicates that no changed data is to be transmitted in the next modification period.
It will be apparent that there is a non-unique mapping from a given service to the transmitted notification indicator; that is, more than one service's notification indicator can generate the same value for q, and consequently be mapped to the same transmitted MICH bits. Consequently, the transmitted MICH bits represent a logical “OR” of the corresponding notification indicators: if none of the notification indicators are asserted (indicating that there is no changed data in the next modification period), then the transmitted MICH bits for those indicators will be equal to “0”.
However, if one or more of the corresponding notification indicators are asserted (indicating that there is changed data for that/those service(s) in the next modification period), then the transmitted MICH bits for all of the mapped notification indicators will be equal to “1”. This means that for the mapped services that do not have changed data in the next modification period the transmitted bits are not accurate indicators of the actual notification indicators.
Consider the following example:
Assume that a UE operating in DRX mode is interested in one service (e.g., a TV Channel). The TV Channel has an identity, the “TMGI” which is used as the identity of the service between the network and the UE. Within an operator's network, three octets are available for uniquely identifying a service, giving the possibility of having as many as 16 million services. The TMGI is used in an equation to derive a 16-bit notification indicator. The notification indicator derived by higher layers of processing is typically provided to a lower Physical Layer. Note that the term Physical Layer is just exemplary and could be implemented in hardware and/or software. The Physical Layer in the UE is responsible for catching the MICH frames (10 ms) when the UE wakes up during a DRX cycle for other activities and checking whether a change indication is set somewhere within the frame. There are n possible places within this 10 ms frame where an alarm could be set for the TV Channel in this example. The Physical Layer uses the notification indicator in an equation to derive a value for q which gives, in the MICH frame, the positions (bits) of the alarm bit that is of interest for this TV Channel. Note that q depends on the SFN, so that the value of q (and hence the position of the alarm within the MICH frame) might change from one frame to another. If the alarm bits are set to 1, then the UE knows that something related to this TV Channel is to be changed in the next modification period and hence the UE has to wake up and read the MCCH channel during the next modification period.
A problem with the procedure described above is that within one MICH frame there can at most be 144 alarm bits. However, as described above, the range of TMGI (about 16 million theoretical possibilities) is far more than 144 services. Because the mapping algorithm does not result in unique mappings, the same alarm bit in the MICH frame could represent one or more services. In other words, the more services an operator introduces the more likely it is that the notification indicators within a frame will overlap in a same alarm bit. As a result, the UE interested in a certain service will suffer from false indications and read MCCH contents in vain although no change has been introduced for the service of interest. The most problematic consequence of this is the unnecessary waste of battery power in DRX mode in which the UE could instead be sleeping. Other problems (not limited to operation in DRX mode) are unnecessary activities affecting processing load, resource conflicts and the like.
It is therefore desirable to provide methods and apparatuses that provide power-efficient monitoring of the alarm bits.